Internal combustion engines are fundamentally known, in which gasoline is directly injected into the combustion chamber and, similar to a diesel internal combustion engine, is self ignited. Additional savings with regard to fuel consumption and once again a more favorable emissions behavior than is the case for internal combustion engines with an externally-supplied ignition via a spark plug can be expected from the self-ignition mode of operation of gasoline engines.
A homogenous self-ignition is thereby achieved in a gasoline engine, in that a significant proportion of the combusted air-fuel mixture is not discharged into the exhaust system but remains in the combustion chamber (so-called internal exhaust gas recirculation). The combusted air-fuel mixture is subsequently referred to as residual gas (RG). In the case of an internal exhaust gas recirculation, residual gas is either kept in the combustion chamber by means of a variable activation of the intake and exhaust valves in the gas exchange-TDC (negative valve overlap), or the residual gas is drawn back out of the exhaust gas port or the inlet port (positive valve overlap).
During the subsequent intake stroke, the residual gas is mixed with combustion air and fuel. In so doing, the fuel mixture situated in the combustion chamber at the point in time of the closing of the one or multiple intake valves, consisting of combustion air and residual gas, has a significantly increased temperature when compared to the normal operation. As a result of the compression of the fuel mixture situated in the combustion chamber, which occurs directly after the intake stroke, the temperature of said fuel mixture increases to such an extent that the fuel-air mixture situated in the combustion chamber ignites without an externally-supplied ignition via a spark plug. It is the goal of the homogenous self-ignition mode of operation that the self-ignition of the fuel-air mixture situated in the combustion chamber approximately takes place when the ignition TDC is achieved.
The residual gas has two important tasks in the process. The hot residual gas first of all provides heat, which in conjunction with the temperature increase during the compression stroke makes a self-ignition of the fuel-air mixture possible.
The second task of the residual gas consists of slowing down the kinetics of the combustion set into motion by the self-ignition. This foremost reduces the mechanical stress on the internal combustion engine and also reduces the engine's noise and prevents the emergence of regional temperature peaks. As a result of this action, an improvement in the degree of efficiency of the internal combustion engine is achieved; and due to the reduced maximum temperatures, an operation of the internal combustion engine, which is very low in NOx content and even partially free of NOx, is made possible.
Such methods are, for example, known from the German patent DE 102 33 612 A1.
The mass of residual gas required for self-ignition increases at low load points/load demands because the exhaust gas temperature drops due to the smaller combusted fuel mass. At higher load demands, a smaller mass of residual gas is required because the exhaust gases are hotter. The temperature of the mass of residual gas remaining in the cylinder, which is load dependant, leads to misfires in the dynamic operation during the change from low to higher loads; while during the change from high to low loads, early combustions occur.